The invention relates generally to subscriber line interface circuits and more specifically to an arrangement in a subscriber line interface circuit for switching the line current between two different DC voltage sources to reduce power losses in the subscriber line interface circuit
A subscriber line interface circuit (SLIC) that is to drive a two-wire transmission line to a load, has a feed characteristic such that the line voltage and the line current are functions of the line load.
When the line is open, i.e. in an on-hook condition of the load, the line is supplied with a maximum voltage at the same time as the current is zero. In a manner known per se, a supply voltage, that is somewhat higher than the required line voltage, has to be available to the SLIC,
When the line is loaded, i.e. in an off-hook condition of the load, the line voltage will decrease and the line current will increase in response to the line resistance plus the load resistance.
The difference between the supply voltage and the line voltage will be applied across the SLIC, The current through the SLIC and the voltage across the same will cause power losses in the SLIC.
The power loss in the SLIC reaches its maximum for short lines, i.e. for low values of line resistance plus load resistance.
In some applications, e.g. so-called NT (Network Terminal) and TA (Terminal Adapter) applications, the lines are extremely short
To keep the power losses low, these short lines should be supplied with as low supply voltages as possible. At the same time, there is a requirement to supply a considerably hither voltage when the line is open, since there may be equipment that needs this higher voltage in order to detect an on-hook condition of the load.
There are solutions where two supply voltages are applied to the SLIC, namely one battery voltage of a lower absolute value to sink the line current in an off-hook condition of the load, and one battery voltage of a higher absolute value to sink the line current in an on-hook condition of the load.
By current-limiting the higher battery in different ways, it has been attempted to control the current to or from that battery,
Another way is to switch the current to the higher battery by means of an internal or external switch when the current has reached a suitable value.
When the current is switched, it is required that the decision level for switching has a hysteresis. Otherwise, one can end up in a situation where the switch just goes on switching between the batteries. Moreover, switching between different voltages results in a step function change of the supply voltage to the SLIC, which causes disturbances.
The object of the invention is to eliminate the problems with the known solutions.
This is attained in that two batteries are connected to the SLIC in accordance with the invention.
One battery of lower absolute value than the other battery is connected to the cathode of a diode whose anode is connected to a voltage regulator connected so that it can be controlled by a control signal. Upon a control signal, the voltage regulator can sink current to the battery of higher absolute value.
When the line voltage is lower than die value of the lower battery, the line cent flows to the interconnection point between the diode and the voltage regulator and via the diode to the lower battery. When the load of the line increases, the line voltage will increase and the line current will decrease, At a certain line current threshold, a control signal will be sent to the voltage regulator so that it begins to sink line current to the higher battery, The diode that is connected to the lower battery, will be reverse biased and all line current flows via the voltage regulator to the higher battery.
If the line voltage continues to increase, the control signal will cause the output terminal of the voltage regulator to follow it accordingly. The output terminal of the voltage regulator will continue to follow line voltage changes until the voltage on its output terminal comes close to the value of the higher battery. There, the voltage regulator will be locked and the line current will be reduced to zero when the line voltage has reached the on-hook voltage.
In this manner, the line current is restricted to the higher battery so that the power losses will be as small as possible. At the same time, the requirements on a high on-hook voltage will be fulfilled. An oscillation, where the line current alternately is switched between the higher and the lower battery, is avoided in that there will be a defined feed characteristic for line currents in the range below the switching threshold,